In electrophotography, a latent electrostatic image is formed by various methods on a photoreceptor made of selenium or some other photoconductive material. The latent image is then rendered visible with toner particles deposited by a suitable development method such as magnetic brush development, and the toner image is transferred onto a receiving element such as paper or sheet and permanently fixed by suitable means such as heat, solvent, or pressure.
Toner image is most commonly fixed by thermal fusing, which is roughly divided into noncontact fusing and contact fusing. Since contact fusing with heated rollers achieves a higher thermal efficiency and enables high-speed fixing, it is used extensively in modern commercial copiers, printers, etc.
However, the method of fixing with heated rollers has several disadvantages. A particularly notable defect is that it consumes a considerable amount of energy or electric power, as compared to fixing by pressure rollers. Needless to say, fixing with heated rollers provides much stronger adhesion between the fixed image and paper or other receiving elements than fixing by pressure rollers, and offers the additional advantage that it prevents deformation or wrinkling of the paper that would otherwise occur upon application of pressure. Therefore, many researchers have conducted studies to find a way to reduce power consumption, or the minimum temperature necessary to fix toner image, when fixing with heated rollers.
One of the most effective approaches for attaining this objective is to reduce the glass transition temperature of the binder resin in a toner by several tens of degrees Celsius lower than the value used customarily. However, many of the toners designed in this way have a fatal defect in that they are prone to caking or agglomeration during storage within a copying machine.
A method was proposed wherein this problem would solved by depositing very fine particles of colloidal silica, alumina, titania, etc. on the surfaces of toner particles so that the latter will have improved anti-blocking and flow properties. This approach was seemingly effective because it achieved some improvement in anti-blocking and flow properties without substantially increasing the minimum temperature necessary to fix toner image. In fact, however, the fine particles mentioned above were found to be readily separable from toner surfaces even when heating or some other treatment was applied to have these particles fused to the latter. The detached particles caused adverse effects on photoreceptors, especially those which were coated with organic polymers, etc. on the surface. In other words, the fine particles separating from toner surfaces would be fixed semi-permanently to the photoreceptor's surface as a result of cyclic operation, causing various image defects. Therefore, depositing fine particles on toner surfaces is not a perfect solution to the problem of caking or agglomeration of toner particles.
Using a styrene acrylic copolymer binder of two-peak distribution which is composed of a low-molecular weight and a high-molecular weight component each having the same monomer ratio has been proposed in U.S. Pat. No. 4,499,168. Because of high miscibility between a high-molecular weight and a low-molecular weight component, however, it is difficult to control anti-offsetting and fixing characteristics independently by this method. If the proportion of the high-molecular weight component is increased, the fixing characteristics are modified for the worse. If the proportion of the low-molecular weight component is increased, the anti-offset characteristics will deteriorate. As a further problem, the fixed image of a toner that employs this styrene acrylic copolymer binder has a tendency to foul polyvinyl chloride (PVC) sheets used as copy files since the styrene-acrylic copolymer has high solubility in a plasticizer in the PVC sheet.
To solve this problem various methods have been adopted and one of them is to use a polyester binder. Polyesters have low solubility in the plasticizer in a PVC sheet and will not cause extensive fouling of the latter. On the other hand, polyesters have a tendency to become negatively charged by triboelectrification, and to produce a toner that is to be positively charged, they must be subjected to one or more treatments that would lead to a substantial increase in toner cost, such as modification, the increased addition of charge control agent, etc.